1. Field of the Invention
The present invention relates to a bandgap reference circuit, and more particularly to a bandgap reference circuit that produces a low sensitivity to the change of temperature and the change of supplied voltage to provide a more stable startup mechanism.
2. Description of Related Art
In the topic of a general bandgap reference voltage, a transistor produces a voltage drop by the positive voltage of a base-emitter voltage (VBE) of a PN junction and adopts the characteristic of the base-emitter voltage being proportional to absolute temperature (PTAT) to produce a reference voltage used for starting up a circuit.
A prior art bandgap reference circuit as disclosed in U.S. Pat. No. 6,788,041 provides a low-power bandgap circuit that produces a reference voltage, and the bandgap circuit comprises a transistor, a bias voltage circuit for producing a bias voltage current, a PTAT current source, and a resistor to provide a low-power and accurate reference voltage, wherein the appropriate resistance of the resistor and the PTAT current can maintain the stability of the current and reduce the sensitivity to temperature. Another prior art as disclosed in U.S. Pat. No. 6,531,857 comprises a plurality of transistors and a low-voltage bandgap reference circuit of the operational amplifiers, or uses a startup circuit to implement the bandgap reference circuit. In U.S. Pat. No. 6,191,644, a better startup circuit is provided, and the present invention further provides a solution for stabilizing voltage and overcomes the shortcomings of a prior art that produces errors during the startup of a circuit.
In FIG. 1, a prior art bandgap circuit comprises identical and gate junction P-type metal oxide semiconductors (PMOS) P1, P2, P3, and a P-type metal oxide semiconductor P1 forming a current mirror transistor of the P-type metal oxide semiconductor P2, P3, and the gates of these three transistors are connected to an input terminal of the operational amplifier OP1, and thus the P-type metal oxide semiconductors P1, P2, P3 have the same drain currents. In other words, I1=I2=I3. In addition, a PNP type bipolar junction transistor (BJT) Q1, Q2 connects its emitter to a drain of the transistor P1, P2 to form a structure similar to a diode, and the emitter area of PNP-type bipolar junction transistor Q2 is an integer multiple N of the emitter area of the transistor Q1.
Further, the control of the same voltage Va, Vb of the input terminal of the operational amplifier OP1 is the same as the drain voltage Va=Vb of the metal oxide semiconductor P1, P2. Since the resistors R1, R2 (R1+R2) having the same characteristics forms a current VBE/R1 passing through the structure, and the base-emitter voltage VBE is inversely proportional to temperature T, wherein the VBE is the base-emitter voltage of the bipolar junction transistors. Since Voltages Va=Vb and the current passing through the resistor R3 is proportional to absolute temperature (PTAT), therefore the PTAT current is equal to VT·1 nN/R3, wherein the VT equals to KT/q, V is the value of voltage, T is the value of absolute temperature value, N is a ratio of emitter areas of the bipolar junction transistors Q2, Q, K is Boltzmann constant, and q is the value of electric charges (in the unit of Coulomb).
The current I2 passing through the transistor P2 is equal to the sum of the current I2 passing through the resistor R2 (=VBE/R1) and the current passing through the resistor R3 (VBE/R3=VT·1 nN/R3), andI2=(VBE/R1)+(VT·1 nN/R3)
and since I1=I2=I3, therefore reference voltage Vref=R4·I3=R4·I2,
Therefore,Vref=R4·((VBE/R1)+(VT·1 nN/R3))
Since Vref=(R4/R3)·((VBE·R3/R1)+(VT·1 nN))
From this formula, if the ratio R3/R1 is an optimum of N, then the reference voltage Vref can produce a low sensitivity to the temperature and supplied voltage.
However, there may be manufacturing errors in the resistors R1, R2, the deviation produced by the operational amplifier OP1 will produce a deviated output of the reference voltage Vref, which will be affected by temperature. More seriously, the manufacturing errors of the resistors R1, R2 of the drains of the transistors P1, P2 will cause an error to the startup of a circuit.
Reference is made to FIG. 2 for the schematic view of the changes of input voltage and output voltage, when the input terminal of the operational amplifier is disconnected with the metal oxide semiconductors P1, P2, P3. In FIG. 2, the inclined straight line shows an output voltage of the operational amplifier OP1 measured when the stable testing voltage is inputted, and the curved line shows a waveform of the voltage when the transistors P1, P2, P3 are disconnected. Three voltage solutions (a, b, c) are observed, and the section (a, b) of the two voltages dropping drastically indicates the value of voltage causing an error to the startup of the circuit, and the point c indicates the value of voltage having a correct startup of circuit. The change of errors of the resistors R1, R2 is shown by the drastic descending waveform between Points a and b. In other words, if the resistor R1 is equal to R2, then the error of the startup of the circuit will not occur.
To overcome the error of the prior art startup circuit caused by the errors of the resistors R1, R2, the present invention provides a more stable startup mechanism to give a better bandgap reference circuit and neglect the errors caused by the resistors, so that the output voltage will not be too sensitive to the change of temperature and the change of supplied voltage.